Abstract

Abstract
The kinetics of sulphur isotope exchange between fine-grained solid sulphides (ZnS, PbS, HgS, sphalerite and galena) and a fluid sulphur phase has been experimentally investigated. The reactions were performed at 250 and 500°C for durations of 0.3 to 5000 h. The experimentally determined time dependence of the isotope distribution in the sulphide phase can be simulated by a combination of three functions. After a rapid change in isotope distribution at the very beginning of the experimental runs, the early stage of exchange can be expressed by the relation: -In (1-F) ∼ r ⋅ t, where F = fractional isotope exchange in the sulphide, r = reaction rate constant, t = time of exchange. This stage of the process is interpreted as a surface reaction. The subsequent slow equilibration of the bulk of the sulphide can be described by a solid state diffusion model. The reaction rate constants and diffusion coefficients obtained for the synthetic systems all follow the Arrhenius relation. Exchange reactions with natural sulphides give lower exchange rates, different equilibrium values, and more scattered plots. These deviations can be explained by lattice distortions caused by impurities in the natural sulphides.
The experimentally derived activation energies for the sulphur isotope exchange in sulphides are within the range 48.2-289.2 kJ/mol, which makes a redistribution of sulphur isotopes probable on geological timescale, even at low temperatures. Natural sulphides show sulphur isotope equilibrium values which differ from those obtained in laboratory experiments with pure sulphides. Inconsistencies in temperature estimates obtained by sulphur isotope geothermometry and fluid inclusion thermometry, for example, can be explained either by the reequilibration of sulphur isotopes in the sulphide or by a shift in the sulphur isotope equilibrium distribution as compared to the standard based on pure sulphides.